1. Field of the Invention
This invention relates to a method of treating a well which has been drilled through an unconsolidated formation in a substantially horizontal direction. In particular, methods are provided for effective flow isolation of selected segments in the substantially horizontal portion of the wellbore by the use of screens and blank pipe and for treating the well to control flow rate of fluids into selected segments of the wellbore.
2. Description of Related Art
In recent years it has become common to drill wells which penetrate the earth in a substantially vertical direction to a selected depth and then deviate from vertical to reach a direction which is substantially horizontal or which is substantially along the bedding planes of the formation being penetrated. The horizontal portion of such wells may extend for as much as hundreds of feet in the same formation. Such horizontal wells are advantageous for producing hydrocarbons from unconsolidated or consolidated formations. Particular advantages may accrue in thin formations, heterogeneous formations, formations having natural fractures in the vertical direction, and in formations where water or gas coning limits the production rate of oil. A horizontal well will normally allow production at a higher rate than a vertical well in the same formation.
A serious limitation of horizontal wells has been the means for "completing" the well. Such completion means preferably allow for controlling or altering the pattern of flow of hydrocarbons and water from selected portions of the wellbore. Some horizontal wells have been drilled and produced from the open hole. This method of production can be practiced in formations where the rock is sufficiently strong for the wellbore to remain open with production, but an open hole provides limited means to alter the pattern of flow from selected portions of the wellbore. In most wells, it is very desirable to be able to alter the pattern of fluid entry into the well. When the horizontal well is drilled through consolidated rock, the casing of the well may be cemented in place and then perforated, which provides a completion means for selectively altering the pattern of fluid entry into the well. When the horizontal well is drilled through unconsolidated rock, a liner made up of pipe containing narrow slots to allow flow into the wellbore (a "slotted liner") or a screen liner is commonly placed in the well. The screen has openings small enough to prevent flow of grains from the formation through the liner and into the wellbore. With the slotted liner or screen liner in the wellbore, it is not possible with conventional methods to control flow rate into the wellbore at different locations along the liner.
The method of cemented and perforated casing or liner is not a suitable method for completing horizontal wells in unconsolidated formations, because the grains of formation rock will flow through the perforations and into the liner, where they will accumulate and retard or prevent flow from the well. An inside-casing screen and gravel pack can be used, but this usually causes an excessively high resistance to flow into the well, does not allow for isolation or control of flow patterns along the wellbore and is an expensive method of completion.
A variety of techniques used in completing horizontal wells have been summarized in the paper "An Overview of Horizontal Well Completion Technology." (SPE Paper No. 17582, Society of Petroleum Engineers, 1988) One of the techniques discussed in this paper is the possible use of uncemented liners having predrilled holes in segments of the liner, these segments being separated by "blank sections." "Blank" sections or "blank" pipe refers to pipe having no holes. The possible collapse of the formation around the blank sections to provide isolation of the predrilled segments is discussed. The method would not be suitable in an unconsolidated or poorly consolidated formation, however, as the grains of the formation could enter the liner through the predrilled holes.
Equipment has been developed which is designed to prevent flow of fluids outside a liner pipe or casing in horizontal wells. A method for completing a horizontal well by employing this equipment, called an "external casing packer," is described in U.S. Pat. No. 4,714,117. This method employs a casing string composed of alternating casing subs and external casing packer subs. The method provides for isolation of discreet segments of the casing string to allow for localized production and remedial treatments in the horizontal portion of the wellbore.
To determine the need for selectively controlling the influx of fluids into a wellbore, flowmeters to measure the flow rate of fluids in a wellbore along the length of the wellbore are available in industry. Also available to be employed with the flowmeter or alone are logging instruments (based on measurements of density or dielectric constant, for example) to determine the relative amounts of gas, water and oil in the flowing stream in the wellbore. In a horizontal well, these flowmeters and logging instruments to determine relative amounts of different fluids may be placed in the well on rigid tubing which can push the instrument along the horizontal portion of the wellbore.
There is a continuing need for an inexpensive method to complete horizontal wells drilled into unconsolidated or poorly consolidated formations so as to allow selective production of fluids from the wells and selective treatment to increase or decrease flow rate from selected segments along the horizontal wellbore.